1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, and more specifically to an apparatus for driving a liquid crystal panel of a relatively large size, which displays characters and pictures.
2. Description of the Prior Art
In recent years, liquid crystals have been extensively used in display devices such as wristwatches, digital clocks, desk-top calculators, and the like. In particular, liquid crystals of the light-receiving type have been placed in extensive applications owing to such features that they can be seen in bright places, they consume small amounts of electric power, and they can be driven on low voltages. Depending upon the operation modes, the liquid crystals can be divided into those of the dynamic scattering effect type (DSM), and those of the field effect type (FEM).
A liquid crystal panel of the dynamic scattering effect type is constructed by, for example, forming an electrode of a transparent electrically conductive film obtained by the evaporation of tin oxide (SnO.sub.2) or the like on a glass plate having a thickness of 0.5 mm to several millimeters, sticking two glass plates together via a spacer, and injecting the liquid crystals into space formed therebetween. The thus constructed liquid crystal panel is usually driven by a matrix driving method. Signal electrodes are arrayed on one side, and scanning electrodes are arrayed on the other side in the directions at right angles with each other. By applying a voltage to the selected signal electrodes and to the selected scanning electrodes, the voltage at the intersecting points exceeds a threshold value, whereby the parallel indicent light undergoes scattering and appears in a whitish turbid color. In this case, the liquid crystal panel is deteriorated within short periods of time if a d-c voltage is applied thereto. Therefore, an a-c voltage is usually applied to the liquid crystal panel.
In the case when the liquid crystal display apparatus which displays characters or pictures, and a control device for controlling the liquid crystal display apparatus, are accommodated in a single housing such as of a pocket calculator, drive signals are applied to the liquid crystal display apparatus simultaneously with the closure of the power source, and a direct current does not flow into the liquid crystals. However, when the liquid crystal display apparatus and the control device are separately provided such as in a monitor television, in which the liquid crystal display apparatus has been designed to be driven by input signals produced by the control device, a direct current flows into the liquid crystal panel when no input signals is applied thereto, presenting such a defect that the display quality is degraded.